1. Field of the Invention
The present invention relates to a wire saw for cutting out many wafers from a work such as a columnar semiconductor ingot, ceramics, glass or the like, and a cutting method using it.
2. Description of the Related Art
Recently, wafers have been required to be large, and highly flat. In order to cope with the large wafer, a wire saw have been mainly used for cutting of an ingot.
A wire saw is an apparatus for cutting many wafers at the same time by a grinding action comprising pressing the work against wires provided with a predetermined pitch, and moving the wire and the work relatively with pouring a cutting fluid containing abrasive grains.
Advantages of the wire saw are that it can cut many wafers at the same time, and thus productivity is high, and that it can produce cut wafers having the approximately same shape owing to simultaneous cutting.
Disadvantage of the wire saw is that a warp (sori) of the cut wafer is large. As a conventional method to solve the problem, there has been adopted a method comprising controlling a temperature of a bearing part of a grooved roller on which the wire is wound to suppress thermal expansion of the roller due to frictional heat during cutting or the like, and thereby the problem of the warp has been solved to some extent.
More specifically, in wire saw frictional heat is generated when the work is pressed against the wire, so that the temperature not only of the work, but also of the working room is increased. If the temperature gets high during cutting, not only the work, but also a part of an apparatus such as a working table is also thermally expanded. As a result, relative position of the work and the apparatus are shifted, and the shape thereof is transcribed to the work as a warp of the wafer.
The conventional method for solving the problem comprises decreasing an influence of increased temperature by applying a cooling medium to a main part of the apparatus such as a bearing, housing or the like. However, there is no means against heat at a part where the work is processed, which is a source of generation of heat, As a result, change in temperature during processing cannot be controlled.
The heat generated during cutting process depends on length of the arc vertical to the direction of cutting (length of the wire that is in contact with the work; cutting length). The change in the length of the arc is large against the direction of cutting. Accordingly, the temperature is significantly changed for a short time after initiation of the cutting, and thus relative shift of the position of the work and the apparatus gets large. The same phenomenon also occurs just before the end of the cutting. Accordingly, the shape having locally large warp formed at the early stage and the terminating stage of the cutting of the wafer (see FIG. 5).
The warp formed during cutting cannot be corrected in the following steps such as lapping, etching or the like, and kept to the end. It has been confirmed that such a warp that is locally large affects flatness during a polishing step.
The present invention has been accomplished to solve the above-mentioned problems, and a main object of the present invention is to provide a method of cutting an ingot and an apparatus therefor wherein a relative shift of the work and the wire is suppressed, a level of a warp of a wafer and a local warp can be improved, and flatness in a polishing step can be improved.
To solve the above-mentioned problems, the present invention is a cutting method comprising winding a wire around plural grooved rollers, and pressing the wire against the work with running it, to cut the work, wherein the work is cut with controlling temperature of the work by supplying a cutting fluid containing abrasive grains to the grooved rollers, and supplying a temperature controlling medium to the work.
As described above, when the work is cut with supplying a cutting fluid containing abrasive grains to the grooved rollers, and supplying a temperature controlling medium to the work, increase of temperature of the work due to the heat generated during cutting of the work can be suppressed to be slow, and the temperature can be kept at a desired value or lower. Accordingly, a level of a warp on the section of the work, a local warp, waviness of all over the work can be improved, and flatness in the following polishing process can be significantly improved. Thereby, productivity and yield of a semiconductor silicon wafer can be improved, and cost performance can also be improved.
The present invention is also a cutting method comprising winding a wire around plural grooved rollers, and pressing the wire against the work with running it, to cut the work, wherein a temperature of the work is previously defined at a predetermined value, and the work is cut with supplying a cutting fluid containing abrasive grains to the grooved rollers.
The method comprises preheating the work to a predetermined temperature before cutting of the work, and then initiating the cutting to cut the work with supplying a cutting fluid containing abrasive grains to the grooved rollers. Thereby, change in temperature of the work, especially at the early stage of the cutting can be made gentle, a level of the warp of the cut surface and a local warp can be significantly improved. If the temperature of the work is increased as described above, it is also advantageous for the reason that the work is hardly affected by external temperature such as room temperature, temperature of the mechanical part of the apparatus or the like.
A method for preheating the work to the predetermined temperature is, for example, a method of preheating the work outside the apparatus, for example using an oven or the like before the work is set in the wire saw, and then set the work therein. Alternatively, there can be adopted the method comprising installing a heater to a plate for holding a work, and heating the work set therein, the method of supplying a temperature controlling medium such as a cutting fluid or air, or the like, controlled in a predetermined temperature to the work and preheating it before cutting.
The present invention is also a cutting method comprising winding a wire around plural grooved rollers, and pressing the wire against the work with running it, to cut the work wherein a temperature of the work is previously defined at a predetermined temperature, and the work is cut with controlling a temperature of the work by supplying a cutting fluid to the grooved rollers, and supplying a temperature controlling medium to the work.
Thereby, the change in temperature of the work in the early stage can be suppressed to be gentle, and increase in the temperature of the work in a period from the middle to the terminating stages of the cutting process can be further suppressed. Accordingly, a local warp generated in the early stages or terminating stages of the cutting process can be made small, and waviness of the whole work and flatness thereof after polishing can be improved significantly.
In that case, change in temperature of the work in a period from the beginning of the cutting process to the time when a cutting length reaches 60% of a diameter of the work and/or in a period from the time when a cutting length reaches 60% of a diameter of the work to the end of the cutting process in the latter half of the cutting is controlled to be 10xc2x0 C. or less.
For example, in the case that the work having a diameter of 8 inches is cut, and the temperature of the work before cutting is about 25xc2x0 C., the cutting length reaches 60% of a diameter when the cutting length in a direction of the diameter is 20 mm after cutting is initiated. Accordingly, the change in temperature of the work in the period should be controlled to be 10xc2x0 C. or less. Namely, the temperature of the wafer at the early stages of the cutting process should be controlled to be 35xc2x0 C. or lower. As described above, when the change in temperature of the work is controlled so as not to be large, especially at the early stages of the cutting process, difference in thermal expansion between the work and the wire saw can be small, and thus extreme change in a shape of the warp does not occur, so that the warp can be made small. In the case that the work having a diameter of 12 inches is cut, the cutting length reach 60% of a diameter when the cutting length in a direction of the diameter is about 30 mm after cutting is initiated. Accordingly, the change in temperature of the work in the period should be actively controlled to be gentle.
In the case that the work having a diameter of 8 inches is cut, the change in temperature of the work is preferably controlled to be 10xc2x0 C. or less in the period until the end of the cutting process after the cutting length reaches 60% of a diameter of the work, namely after the remaining cutting length is about 20 mm, since the warp can be made small as in the early stages of a cutting process.
As described above, it is preferable to make change in temperature of the work gentle in the early stages and in the terminating stages, since change in temperature during cutting can be suppressed thereby.
In that case, the temperature of the work can be predetermined so that a shape of the warp of the wafer defined by simulation with coefficient of linear expansion and temperature of each part of the work and the wire saw can be flat.
As described above, it is simple and convenient that the temperature of the work to be controlled during a cutting process is defined by simulation. In the present invention, the data as for the warp obtained by the simulation are well consistent with the actual data.
In that case, the above-mentioned temperature controlling medium can be a cutting fluid of which temperature is controlled and/or an air of which temperature is controlled.
As described above, the temperature of the work can be controlled by directly pouring a cutting fluid of which temperature is controlled to be the constant value as a temperature controlling medium to the work, or by spraying an air of which temperature controlled to be the predetermined value to the work. It is especially simple and preferable to supply a cutting fluid to the work, since a structure of an apparatus can be simple, and a fluid after cutting can be easily collected. It is also possible to use both of the method of pouring a cutting fluid and the method of spraying air.
The temperature of the work during cutting is preferably kept lower than 35xc2x0 C.
As described above, if the work is cut with supplying a cutting fluid containing abrasive grains of which temperature is, for example, about 25xc2x0 C. to grooved rollers, and with supplying a temperature controlling medium of which temperature is controlled directly to the work so that temperature of the work during cutting can be kept at lower than 35xc2x0 C., the temperature of heat generated at the cutting part can be suppressed, thermal expansion of the wire saw and the work can be made small, a shift of the relative position between the work and the wire is also small. As a result, level of a warp on the cut surface of the work, a local warp formed in the early stage or the like, waviness that is a shape of the whole work, and flatness can be improved. Particularly, if the temperature controlling medium is supplied directly to the work, the temperature of the work can be controlled accurately and easily. The temperature of 35xc2x0 C. to which the work should be controlled during cutting is defined according to the above-mentioned simulation.
It is desirable to control the temperature of the plate part for supporting the work.
If the temperature of the plate part for supporting the work is controlled, and the temperature of the work is indirectly controlled, distortion such as expansion or the like of the plate part can be suppressed. Such a method is further effective to improve the warp of the work.
The present invention is a wire saw wherein a wire is wound around plural grooved rollers, and a work is cut by pressing the wire against the work with running the wire, comprising a means for supplying a cutting fluid containing abrasive grains of which temperature is controlled to the grooved rollers, a means for pouring a cutting fluid containing abrasive grains of which temperature is controlled directly to the work or a means for spraying a medium, especially air of which temperature is controlled directly to the work.
If the wire saw has such a constitution, temperature of the heat generated through a cutting process from initiation to the end thereof can be kept low, change during cutting due to thermal expansion of a work or a wire saw gets small, and a semiconductor wafer wherein a warp can be kept small and approximately constant can be provided by the wire saw.
In that case, the above-mentioned wire saw can be equipped with a temperature controlling means at a plate part for supporting the work. Namely, a temperature controlling means such as a heater, a heat exchanger or the like can be provided at the plate part to conduct heating and cooling.
If the wire saw is constituted as described above, and the temperature of the plate part itself supporting for the work is controlled, a deviation due to thermal expansion at the plate part can be prevented, further high cutting accuracy can be achieved, so that the wire saw can provide a work having further small warp. It can also be used as a means for preheating a work.
As described above, according to the present invention, difference in thermal expansion between a work and a wire saw becomes small, extreme change in the shape in the early stage of the cutting process can be prevented, the warp can be made small, and thus a wafer having a desired shape of a warp can be cut out. Accordingly, flatness is hardly affected in the later polishing step. By simulating a shape of a warp, an adequate condition for cutting can be selected, productivity and yield in a cutting process of a semiconductor silicon ingot can be improved, so that cost performance can be greatly improved.